wifi: mt76: mt7921e: add LED control support

Introduce wifi LED switch control, add flow to Control a wifi
gpio pin based on the status of WIFI radio, if the pin is connected
to an LED, the LED will indicate the status of the WiFi radio.

Signed-off-by: Hao Zhang <hao.zhang@mediatek.com>
Co-developed-by: Quan Zhou <quan.zhou@mediatek.com>
Signed-off-by: Quan Zhou <quan.zhou@mediatek.com>
Acked-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>